Acid-Modulated Synthesis of Novel π-Conjugated Microporous Polymers for Efficient Metal-Free Photocatalytic Hydrogen Evolution.

Abstract

Simple synthetic modifications that tune the molecular structures, thereby the properties of the molecules, are of topical interest. Herein, we report the synthesis of two novel cationic rosaniline-based conjugated microporous polymers (CMPs) from identical monomers via simple acid modulation (Acetic acid and BF3 ⋅ Et2 O). The condensation reaction of rosaniline with 2,4,6-triformylphloroglucinol in acetic acid renders β-ketoenamine-linked CMP (CMP-A) while changing the acid to BF3 ⋅ Et2 O, the linkages transform to enol and undergoes BF2 -complexation, leading to boranil CMP (CMP-B). BF2 -functionalities in boranil CMP significantly modified the optical and functional properties compared to β-ketoenamine-linked CMP. The cationic-delocalization along with the extended π-delocalization supported by chromophoric BF2 -groups allow CMP-B to exhibit broad absorption spanning the visible to Near-Infrared region (NIR). The absorption red-edge of CMP-B appears around 1277 nm (optical band gap ∼1.58 eV) while CMP-A displays at 981 nm (optical band gap ∼1.83 eV). Most interestingly, as a photocatalyst, CMP-B catalyzes hydrogen evolution with a superior rate of 252 μmol g-1 over CMP-A (100 μmol g-1 ). It is about 2.5 times higher performance. The transient photocurrent measurements, electrochemical impedance data, and in-depth mott-Schottky analysis demonstrate that the BF2 -group in CMP-B generates photoinduced charge carriers and their migration towards the active sites for photocatalysis. These polymers show significant photocatalytic H2 generation without any supportive metal co-catalyst. The BF2 complexed building blocks are a unique class of metal-free photocatalysts for hydrogen evolution through green and cost-effective approach.